MAE 155B: Aerospace Engineering Design II Lecture 5: Propulsion John T. Hwang Winter 2019 aircraft and engine manufacturer production lines since 1989. Historically, propulsion contributed We conclude that fuel costs alone have nothas been sufficient to stimulate increased aircraft efficiency, and that improvementshave in fuel efficiency due to the introduction Propulsion advancements contributed Fuel burn have stagnated since thethat 90sa CO of newhalf aircraft reductions have decreased over time. These findings suggest about of fuel burn reductions built reductions aircraft from current production lines, not just aboutstandard halfthatofapplies fuelto newly burn 2 Fuel burn at design range (1960=100) ! to new designs, is most likely to reduce emissions. 100! 1960s! Annual Improvement ! Period Seat-km Ton-km! 1960s 2.3% 3.6%! 1970s 0.6% -0.1%! 1980s 3.5% 2.5%! 1990s 0.7% 0.9%! post-2000 0.0% 0.3% 1970s! 75! seat-km! 1980s! 1990s! ton-km! post-2000! 50! 25! 1960! 1965! 1970! 1975! 1980! 1985! 1990! 1995! 2000! 2005! 08! Year! ABSTRACT 1. AVERAGE FUEL BURN FOR NEW AIRCRAFT, 1960-2008 [Efficiency Trends for New Commercial Jet Aircraft. ICCT, 2009] (Rutherford and Zeinali 2009) 2 John T. Hwang (University of California San Diego) 2 Basic thrust analysis 476 Aircraft Design: A Conceptual Approach v (Raymer Daniel 1999) Fig. 13. 1 Simplified thrust analysis model. While we often write our equations as if the world is a wind tunnel with the airplane stationary and the air coming at it, the reality is the opposite. This model represents a generic aircraft propulsion system-a perfect propeller or a very short jet engine-which is actually flying through the air at velocity Vo and accelerating the air it encounters by a change in velocity equal to (V- Vo). Subscript zero indicates the freestream condition, and the mass flow of air passing through the disk is easily found as air density times velocity times cross-section area S. Newton's equation, redefined for fluid flows, states that the force produced equals the mass flow rate times the applied change in velocity, leading to Eq. (13.1). The rate of useful work done by the propulsion system, called the thrust power Pt, equals the product of the thrust force and the aircraft velocity [Eq. (13.2)]. The change kinetic energy (i.e., work) imparted to the fluid by the John T. Hwang (University of California San in Diego) propulsion system is determined by the difference in fluid velocity. 3 Basic thrust analysis 476 Aircraft Design: A Conceptual Approach v (Raymer Daniel 1999) Fig. 13. 1 Simplified thrust analysis model. While we often write our equations as if the world is a wind tunnel with the airplane stationary and the air coming at it, the reality is the opposite. ConservationThis ofmodel momentum—thrust is equal to perfect masspro-flow rate represents a generic aircraft propulsion system-a peller or a very short jet engine-which is actually flying through the air at times the change inandspeed (we’ll this Week 8): velocity Vo accelerating the air derive it encounters by a in change in velocity equal to (V- Vo). Subscript zero indicates the freestream condition, and the mass flow of air passing through the disk is easily found as air density times velocity times cross-section area S. Newton's equation, redefined for fluid flows, states that the force produced equals the mass flow rate times the applied change in velocity, leading to Eq. (13.1). The rate of useful work done by the propulsion system, called the thrust power Pt, equals the product of the thrust force and the aircraft velocity [Eq. (13.2)]. The change kinetic energy (i.e., work) imparted to the fluid by the John T. Hwang (University of California San in Diego) propulsion system is determined by the difference in fluid velocity. T = ṁ∆V 3 Basic thrust analysis 476 Aircraft Design: A Conceptual Approach v (Raymer Daniel 1999) Fig. 13. 1 Simplified thrust analysis model. While we often write our equations as if the world is a wind tunnel with the airplane stationary and the air coming at it, the reality is the opposite. ConservationThis ofmodel momentum—thrust is equal to perfect masspro-flow rate represents a generic aircraft propulsion system-a peller or a very short jet engine-which is actually flying through the air at times the change inandspeed (we’ll this Week 8): velocity Vo accelerating the air derive it encounters by a in change in velocity equal to (V- Vo). Subscript zero indicates the freestream condition, and the mass flow of air passing through the disk is easily found as air density times velocity times cross-section area S. Newton's equation, redefined for fluid flows, states that the force produced equals the mass flow rate times the applied change in velocity, leading to Eq. (13.1). 0The rate of useful work done by the propulsion system, called the thrust power Pt, equals the product of the thrust force and the aircraft velocity [Eq. (13.2)]. The change kinetic energy (i.e., work) imparted to the fluid by the John T. Hwang (University of California San in Diego) propulsion system is determined by the difference in fluid velocity. T = ṁ∆V T = ṁ(V − V ) where ṁ = ρSV 3 Basic thrust analysis T = ṁ(V − V0 ) where ṁ = ρSV John T. Hwang (University of California San Diego) 4 Basic thrust analysis T = ṁ(V − V0 ) where ṁ = ρSV Pout = TV0 John T. Hwang (University of California San Diego) 4 Basic thrust analysis T = ṁ(V − V0 ) where ṁ = ρSV Pout = TV0 Pout = ṁ(V − V0 )V0 John T. Hwang (University of California San Diego) 4 Basic thrust analysis T = ṁ(V − V0 ) where ṁ = ρSV Pout = TV0 Pout = ṁ(V − V0 )V0 Pin = ∂ ∆E ∂t John T. Hwang (University of California San Diego) 4 Basic thrust analysis T = ṁ(V − V0 ) where ṁ = ρSV Pout = TV0 Pout = ṁ(V − V0 )V0 ∂ ∆E ∂t 1 1 Pin = ṁV 2 − ṁV02 2 2 Pin = John T. Hwang (University of California San Diego) 4 Basic thrust analysis T = ṁ(V − V0 ) where ṁ = ρSV Pout = TV0 Pout = ṁ(V − V0 )V0 ∂ ∆E ∂t 1 1 Pin = ṁV 2 − ṁV02 2 2 1 Pin = ṁ(V + V0 )(V − V0 ) 2 Pin = John T. Hwang (University of California San Diego) 4 Basic thrust analysis T = ṁ(V − V0 ) where ṁ = ρSV Pout = TV0 Pout = ṁ(V − V0 )V0 ∂ ∆E ∂t 1 1 Pin = ṁV 2 − ṁV02 2 2 1 Pin = ṁ(V + V0 )(V − V0 ) 2 Pin = η= Pout 2 = Pin V /V0 + 1 John T. Hwang (University of California San Diego) 4 Basic thrust analysis η= 2 Pout = Pin V /V0 + 1 T = ṁ(V − V0 ) where ṁ = ρSV John T. Hwang (University of California San Diego) 5 Basic thrust analysis η= 2 Pout = Pin V /V0 + 1 T = ṁ(V − V0 ) I where ṁ = ρSV We see that V > V0 in order to generate thrust. John T. Hwang (University of California San Diego) 5 Basic thrust analysis η= 2 Pout = Pin V /V0 + 1 T = ṁ(V − V0 ) where ṁ = ρSV I We see that V > V0 in order to generate thrust. I As V → ∞, η → 0. John T. Hwang (University of California San Diego) 5 Basic thrust analysis η= 2 Pout = Pin V /V0 + 1 T = ṁ(V − V0 ) where ṁ = ρSV I We see that V > V0 in order to generate thrust. I As V → ∞, η → 0. I As V → V0 , η → 1. John T. Hwang (University of California San Diego) 5 Basic thrust analysis η= 2 Pout = Pin V /V0 + 1 T = ṁ(V − V0 ) where ṁ = ρSV I We see that V > V0 in order to generate thrust. I As V → ∞, η → 0. I As V → V0 , η → 1. I Therefore, we want V to be as small (close to V0 ) as possible for maximum propulsive efficiency. John T. Hwang (University of California San Diego) 5 Basic thrust analysis η= 2 Pout = Pin V /V0 + 1 T = ṁ(V − V0 ) where ṁ = ρSV I We see that V > V0 in order to generate thrust. I As V → ∞, η → 0. I As V → V0 , η → 1. I Therefore, we want V to be as small (close to V0 ) as possible for maximum propulsive efficiency. I This means that, for the same amount of thrust T , S must be as large as possible. John T. Hwang (University of California San Diego) 5 AE481 — Martins Sunday 13th November, 2011 at 23:22 71 196 5 Aircraft Engines and Propulsion Fuel-based propulsion—4 types for subsonic map 278 Aircraft Design: A Conceptual Approach Figure 5.8 Figure Classification of engine concepts mostly used in aviation. 11.1: Classification of propulsion systems Burner Compressor I Turbine Burner Compressor its lower density when compared to kerosene requires that isI \ a tank capacity \ four times larger for flying the same distance. Furthermore, the production Figure 5.8 Figure Classification of engine concepts mostly used in aviation. and distribution will require the development of new infrastructure. 11.1: Classification of propulsion systems 1. Piston Piston-prop Centrifugal turbojet Turbine I Axial-flow turbojet its lower 5.2 density when compared to kerosene requires a tank capacity that is Fundamentals of reaction propulsion four times larger for flying the same distance. Furthermore, the production Bypass air Flameholders Engine will concepts and distribution require the development of new infrastructure. Fuel spray barsl Figure 5.8 Figure Classification of engine concepts mostly used 11.1: Classification of propulsion systems in aviation. Turbojet < Figure 5.8 shows a schematic overview of the most important engine orcon- <0 turbofan < cepts used in aviation. Except the rocket engine, all of them are air breathing lower density compared to kerosene requires a tankcategories capacity that is 5.2 its Fundamentals of propulsion engines that canwhen be reaction used for Turboprop atmospheric flight only. The following Turbofan Afterburner four are times larger for flying the same distance. Furthermore, the production distinguished: and distribution will require the development new infrastructure. Fig. 10.1 of Propulsion system options. • Piston engines. Engine concepts 2. Turbojet 3. Turbofan • Turboprop and turboshaft engines. • Turbojet engines, also called straight or simple jet engines. of the engine andof its the inlet most duct orimportant propeller. Also, the fuelconsystem must be Figure 5.8 shows a schematic overview engine • Turbofan engines. defined, especially the fuel tanks that carry a large fraction of the total Fundamentals of reaction propulsion cepts5.2 used aviation. Except the rocket • inRamjet engines. aircraft weight. engine, all of them are air breathing This chapterflight treats the integration and layout of the propulsion system engines that can be used for atmospheric only. The following categories This scheme is not only based on mechanical differences betweenofpropeller the overall vehicle design. The calculation installed propulsion perinto Engine concepts are distinguished: and jet propulsion, but also the energy conversion process has been taken formance is covered in Chapter 13 . 4. Turboprop into account. The different types will be discussed in the following sections, • Piston engines. Figure 11.2: Diagrams showing the components for the turbojet, turboprop and turbofan; note that all three have a gas generator in common. which also adiscuss the technological implementation of the firstengine four conFigure 5.8 will shows schematic overview of the most important • Turboprop and turboshaft engines. categories mentioned above. Gas turbine engines with reheat and ramjet enPropulsion Overview and Selection cepts used in aviation. Except the rocket engine, all of them are air breathing • Turbojet also called straight or simple jet engines. ginesengines, will be discussed in Chapter 9. Besides the engine types mentioned in 10.1 illustrates the only. major The options for fuel-based aircraft propulJohn T. Hwang (University of California Diego) engines that can San be used forFigure atmospheric flight following categories • Turbofan engines. sion. These all operate by compressing outside air, mixing it with fuel, burnare distinguished: .m 6 because they provide more thrust per unit power, but electric fans are also in use. Electric propulsion is discussed at the end of this chapter. Fuel-based propulsion (Typical applications) / u u.. ..--- Vl 0\ c ·v; / "'u / c --- --- Rocket ? Sc ramjet Ramjet Afterburning turbojet Afterburning low-bypass-ratio turbofan Low-bypass-ratio turbofan 1 High-bypass-ratio turbofan 1=7" Turboprop l=7 Piston-prop 0 2 3 4 5 6 Design Mach number John T. Hwang (University of California San Diego) (Raymer Daniel 1999) 7 Piston engine First type of aircraft propulsion used: I Similar to automobile engine I But air-cooled, lighter, more reliable I Efficient, cheap; but heavy, noisy I Larger props more efficient, but heavier and require more clearance John T. Hwang (University of California San Diego) 8 Piston engine The piston engine is limited to subsonic speeds: I Limited by helical tip speed due to the possibility of shocks I Blade angle of attack decreases as forward speed increases I High pitch causes stall at low speeds I This motivates variable-pitch props John T. Hwang (University of California San Diego) 9 Piston engine The piston engine is constant-power: Power: I Proportional to RPM I Constant in velocity Linearly increases with density Brake-specific fuel consumption (BSFC) I Fuel burn rate per unit power I I Constant in velocity and density John T. Hwang (University of California San Diego) 10 Turbine engines Turbine engines are constant-thrust: I Four steps: compress air; mix it with fuel; burn the mixture; extract energy from the high-pressure hot gas I Pros: can operate at higher speeds I Cons: not as efficient, especially at low speeds (small, fast exhaust) John T. Hwang (University of California San Diego) 11 Fuel-based propulsion Turbojet engines: I Compressor: compress air to many times atmos. pressure. I Burner: inject fuel, mix it with air, and ignite the mixture. I Turbine: before expelling the hot gas, pass it through a turbine to drive the compressor. Turboprop engines: I Idea: add conventional propeller to turbojet. I Prop-fan or unducted fan: advanced propellers—loud. I Open rotor: turbofan-like props—efficient but loud. Turbofan engines: I Idea: turbine drives a ducted fan. Some of the air bypasses the core, and the rest is ducted into the turbojet core. I Quieter: the duct suppresses noise, and the thrust from the bypassed air enables a reduced exit velocity because of reduced strength of noise-generating shear layers. John T. Hwang (University of California San Diego) 12 (TSFC) — is the fuel consumed by unit time for each unit of thrust. In English units, SFC is in pounds of fuel per hour, per pound of thrust. SFC varies with Mach number, throttle setting and altitude. Fig. 2.2 show the typical variation of SFC with Mach number for various types of engines. Fuel-based propulsion TSFC vs. Mach Number John T. Hwang (University of California San Diego) Figure 2.2: SFC vs. Mach number for various engines Mattingly [2, Fig. 1.17b] 13 Electric propulsion I No emissions, and more than 90% energy efficiency possible versus only 20% for gas engines I Fewer moving parts than reciprocating or turbine engines, so greater reliability and simplicity I Smooth and quiet I Power does not decrease with altitude I Can be overpowered for a short time for takeoff or emergency conditions I Gearbox is not needed John T. Hwang (University of California San Diego) 14 The blade angle of attack is a key aspect of the propellor design Propeller-driven aircraft 9.2 Propeller Tip Voo I Section A-A Views looking down from the top r L_ Root Voo Side view Section B-B Figure 9.3 Illustration of propeller, showing variation of pitch along the blade. John T. Hwang (University of California San Diego) 15 p Propellor-driven aircraft are shaft of the engine) and PAis the power available from the propeller. As giv Propeller-driven aircraft Eq. (6.31), PA = TA Voo. Hence Eq. (9.2) becomes fundamentally limited in speed TAVoo 9.2 Propeller 17=----p where P is the shaft brake power (the power delivered to the propeller b C H A PT E R 3 • Some Propulsion Characteristics 157 7) rw As previously explained, TA in Eq. (9.3) is basically an aerodynamic 1.0 - - - - - - - - - - - - - - - - - - - - - - - - nomenon that is dependent on the angle of attack a in Fig. 9.5. In turn dictated by the pitch angle fJ and ¢, where ¢itself depends > >on the magnitud V and rwrOJ. The angular velocity OJ= 2;r n, where n is the number of pro revolutions per second. Consequently, TA must be a function of at least f and n. Finally, the thrust must also depend on the size of the propeller, char ized by the propeller diameter D. In turn, the propeller efficiency, from Eq. must depend on fJ, Voo, 17, and D. Indeed, theory and experiment Voo both show J=(b) nD for a fixed pitch angle fJ, 17 is a function of the dimensionless quantity 00 Velocity and relative wind diagrams For a section of a revolving propeller: (a) Case For low V00 and (b) case for high V00 • Advance ratio Figure 9.6 Propeller efficiency versus advance ratio. Note that D denotes J= Voo propeller diameter. advance ratio nD Propeller These losses occur because of several different effects. First imagine that you RPM typical variation 17 with J for is no sketched Fig. 9 .6; three c are standing in anof open field. The airaisfixed still;diameter it has velocity. in Then a propeller{3. The angle of attack a is theAangle between the chord wind. The angle of attack clearly on the relativegoes zooming driven vehicle by you.different After the propeller haspitch. passed,Figure you will 9.6 feel is im aredepends shown corresponding to three values of ig. 3.6a, V00 is small, and a is a fairly large positive value, afrom stiff breeze moving in17 the of the vehicle. This breeze tant; curves is direction obtainedopposite for anthat airplane performance analys c "lift" L acting in the general thrust direction. In such Fig. 3.6b, is part of the slipstream from the propeller; that is, the air is set into both transly increased; all other parameters remain the same. Here,6. described inCh. lational motion by the passage of the propeller. Consequently,16 Hwang of California Sanand Diego) ved John to theT.other side(University of the airfoil section, giving riserotational to Examine Fig. more closely. Note that 17 < energy 1; this of is the because some you observe some and rotational kinetic air where dynamic lift force L pointing in the opposite direction of 9.6translational fJ Computing thrust and torque Fig.22 Propeller Propeller Blade Geometry Fig. Blade Geometry Fig. 3 and Scatterplot of the surrogate model inputs Lee, and Fig.(Ha, 3 Scatterplot of theHwang surrogate2019) model inputs CQ = John T. Q 1 2 2 ⇢V Sr , (5) where ⇢ is equal to air density, V is the airspeed, S is the propeller blade area, and r is the propeller blade radius. and The SMT is trained with these dependencies and outputs and is visualized in Fig. 4, accurately portraying how as Q must decrease accordingly by increasing RPM, thus blade angle of attack increases a fixed pitch angle,Cinflow velocity Hwang (University of California San atDiego) , Q = 2 Sr increasing both torque and thrust. Separate design variables 1are for each propeller while mirrored pairs of propellers ⇢Vset 2 (5) 17 1.0 1.0 0.8 0.8 Thrust coefficient Torque coefficient Computing thrust and torque 0.6 0.4 0.2 Blade angle of attack ( ) =3 =2 =1 =0 = -1 = -2 = -3 0.6 0.4 0.2 0.0 0.0 5 Fig. 4 10 15 20 Blade pitch angle ( ) 25 5 10 15 20 Blade pitch angle ( ) 25 Lee, andblade Hwang 2019) Coefficient of torque (Ha, and thrust versus pitch angle at several blade angles of attack. since dCMi / d↵ = 0, and thus neutral point can be calculated by 1 T = CT ρV 2 S 2 x np 1 and Q = CQ ρV 2 Sr 2 Õ dCLi Si xaci i d↵ = . Õ dCLi Si i d↵ (8) Using the neutral point and the center of gravity we can calculate the static margin, which we add as a constraint to ensure longitudinal stability. John T. Hwang (University of California San Diego) 18 References i Ha, Tae Hyun, Keunseok Lee, and John T Hwang (2019). “Large-scale design and economics optimization of eVTOL concepts for urban air mobility”. In: 2019 AIAA/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference. doi: 10.2514/6.2019-1218. Raymer Daniel, P (1999). “Aircraft Design: A conceptual approach”. In: AIAA Education series. Rutherford, Daniel, Mazyar Zeinali, et al. (2009). “Efficiency trends for new commercial jet aircraft 1960-2008”. In: John T. Hwang (University of California San Diego) 19